ABSENCE OF INTERSPECIFIC COMPETITION AMONG TALLGRASS PRAIRIE GRASSHOPPERS DURING A DROUGHT

The grasshoppers Phoetaliotes nebrascensis and Orphulella speciosa overlap extensively in both diet and habitat in native tallgrass prairie at Konza Prairie, Kansas. I tested for interspecific competition between these two species during a drought year when primary productivity in the prairie was unusually low. In late June 1989, 1 transferred young nymphs of P. nebrascensis among enclosed plots (each 550-900 m2) to produce three pairs of plots with grasshoppers removed or added. Pretransfer densities in the six plots were estimated at 6 nymphS/m2. Experimental transfers resulted in densities of P. nebrascensis in addition plots 2-3 times as large as in paired removal plots; significant differences in density between addition and removal plots persisted throughout the summer. Densities of O. speciosa in plots with P. nebrascensis removed or added did not differ significantly throughout the growing season of 1989, nor in early summer of 1990. Furthermore, growth and development of O. speciosa in these plots (as measured by rate of maturation, and adult femur length and mass in 1989) were not reduced in the presence of high densities of P. nebrascensis, nor was reproductive potential (measured as the ratio of young nymphs present in early summer 1990 to adults in late summer 1989). These results are similar to those of a previous experiment at the same site during 1987 when primary productivity was moderate. Even under the extreme conditions of 1989, with particularly high ratios of grasshoppers to available food resources, variation in P. nebrascensis density did not adversely affect populations of O. speciosa. These results contrast with recent results for sedentary phytophagous insects, but are consistent with the hypothesis that free-ranging insect herbivores may rarely experience interspecific competition for food resources.